In one prior art, U.S. Pat. No. 6,719,107, a power generator is used to drive a motor in a gear box. It has a slow reaction to resisting force. For example, about 20 seconds are necessary for adjusting from a Level 1 to Level 9. If we reduce this time, the resolution for precise become worse.
In another prior art, U.S. Pat. No. 6,084,325, a current driving motor serves to adjust the resistance force with a quick response and the resolution of the current to control the resistance force is higher. In U.S. Pat. No. 6,719,107, the gear 33 in the gearbox is directly connected to a rod 34. The rod 34 is further connected to the rod 21. The gearbox has a variator (variable resistor) for detecting the resisting force. The parts of this patent are easy to destroy as it is used for a long time. In repairing work, the whole gearbox must be detached and then the rod 34 is detached for updating the parts. The repair work is inconvenient.
In another prior art U.S. Pat. No. 5,711,404, two ropes 72 are used to pull the brake pieces 51. The ropes 72 are soft. If the gap between the magnet of the brake and the magnetic isolation ring is overlarge to be out of the control of the absorption force of the magnet, the brake piece cannot return to the normal position. Thus a spring 53 is used to assist the returning work of the brake piece 51.
It should be noted that the numerals of the elements cited above are identical to the specification of these patents. Thus the readers can understand these patents directly from the specifications of these patents.